apt-get update 升级错误

W: GPG error: http://mirrors.cloud.aliyuncs.com/ubuntu xenial-backports InRelease: Could not execute ‘apt-key‘ to verify signature (is gnupg installed?)
W: The repository ‘http://mirrors.cloud.aliyuncs.com/ubuntu xenial-backports InRelease‘ is not signed.
N: Data from such a repository can‘t be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.

因为tmp 文件夹权限不够

chmod 777 /tmp

apt-get update --allow-unauthenticated

参考：https://blog.csdn.net/gpf19930310/article/details/82461306